The need for light weight, inexpensive heat exchange elements for various heat transfer applications has been in demand by industry for a long time. The automobile industry has constantly been searching for a compact, light weight radiator for use in cooling the internal combustion engine. Various types and styles of radiators have been designed, such as the individually finned round tubes, the hexagon-shaped air tubes with water passages between the tubes, and the flat dimpled water passages with air flow therebetween. The pre-1942 automobile engines were designed to deliver between 50 and 125 horsepower and required radiators operable close to atmospheric pressure. A simple solder joined finned-copper constructed radiator was therefore sufficient to cool the low horsepower engine of the automobile without much of a threat of overheating. Various copper radiators having cup-like or frustro-conical surface projections have been designed during this pre-1942 period but the finned copper radiator proved more successful and suitable for automobile applications.
The automobile industry, however, in the post-1945 era embarked upon the design of higher power rated engines while simultaneously attempting to compact them as much as possible. This dual design approach coupled to the employment of improved lubricants resulted in an internal combustion engine capable of operating at high permissible temperatures. To satisfy the heat transfer requirements of such compact high power rated engines and to avoid loss of coolant, the tube and fin copper radiators were designed to operate under pressure so as to increase the boiling temperature of the coolant. However, within the last several years, additional power operated equipment, such as air conditioners and the like, was added to the automobile thereby further increasing the demands on the internal combustion engine and consequently the duty of the heat rejection system. This has necessitated the designing of present day radiators to operate at pressures as high as 15 psig to prevent coolant loss and overheating. The operating temperature of the automobile engine is anticipated to rise further in the near future thereby necessitating a heat transfer system operable with existing coolants under still higher pressure conditions. The conventional type finned-copper radiator will not perform satisfactorily in an increased temperature environment due to the low stress characteristics inherent in soft solder at high temperatures, such solder being the securing medium between the tubes and fins of the radiator. In addition, the steady increase in the price of copper is causing copper to become an undesirable material for radiator applications from an economical standpoint.
An alternate solution to the conventional type finned-copper radiators for heat transfer applications in the automobile, is to replace the soft soldered copper fins with aluminum fins. Although aluminum is less expensive than copper, the fusion bonding of aluminum fins to the tubes of a conventional radiator by brazing techniques is expensive. In addition, if a corrosive flux is used, the deposits left by the salt bath of the brazing process must be meticulously removed. Alternate brazing techniques and methods, i.e., vacuum brazing, are still in the experimental stage and when perfected their high cost will probably overwhelm the savings otherwise gained in the use of aluminum rather than copper for producing automobile radiators. Other proposals have been advanced, such as the use of adhesive bonding between the fins and tubes of a radiator. However, the low thermal conductivity of present day adhesives renders this approach inefficient for radiator applications.
In heat exchange applications requiring pressure-bearing walls as the primary heat exchange surface, the present invention enables such walls to be fabricated from thinner thermally conductive material than is presently required of conventional type primary heat exchangers. In order to utilize relatively thin sheet materials, the walls of conventional type primary heat exchangers have to be stayed by means of numerous support members so as to reduce stress in the walls. However, stayed walls are normally not practical because of the following reasons:
(a) high stress concentrations are still produced in the wall at the point of attachment of the stays;
(b) a substantial amount of material is required in the stays, and in heat exchangers such stays contribute only indirectly if at all to heat transfer; and
(c) the numerous stays are tedious and expensive to install, particularly in heat exchangers where the spacing between walls is very small and often inaccessible. The present invention overcomes the above drawbacks by providing an isostress contoured heat exchange surface which upon being subjected to a differential pressure across its wall will result in a substantially uniform fiber stress distribution in the wall. This uniform stress distribution substantially eliminates stress concentration points in the wall of a heat exchange element thereby permitting the element to be fabricated from rather thin sheets of thermally conductive material.
Another approach to the elimination of stayed walls in primary surface heat exchangers is disclosed in U.S. application Ser. No. 189,509 titled "Primary Surface Heat Exchanger," filed Oct. 15, 1971 in the names of L. C. Kun and J. B. Wulf and issued Sept. 11, 1973 as U.S. Pat. No. 3,757,855, which relates to dimensionally sized and spaced truncated-conical projections in thermally conductive walls.
The present invention is directed to a method of making a die suitable for use in manufacturing an all-purpose, primary-surface heat exchange channelized element having on at least a portion of its surface isostress contours with substantially uniformly disposed unidirectional wall-supporting projections. The heat exchange element is economical to fabricate and when employed in stacked units, they are admirably suited as a heat exchanger for use with internal combustion engines.